The present application generally relates to motor control systems, and particularly to preventing windup in motor control systems, and more particularly to those used in systems like electric power steering (EPS) systems.
EPS systems typically use an electric drive to provide steering assist torque to a driver. Typically, torque control of an electric drive system that uses a Permanent Magnet Synchronous Machines (PMSMs) is performed indirectly by regulating the current. In general, the current control of machine currents is performed using a feedback control architecture with measured currents in the synchronously rotating reference frame utilizing Field Oriented Control (FOC) techniques, such as feedback control closed-loop. Feedback control in general has good steady-state tracking performance, dynamic response, high bandwidth and satisfactory disturbance rejection. As a result, feedback current control is typically used to control multi-phase AC machines, such as the PMSMs.
Closed-loop current control of electric motors in EPS systems has demanding requirements outside of the control system's capability to track the desired assist torque command (e.g. motor torque command). Among these, consistency of performance throughout the operating range of the control system is required, including operation throughout the motor velocity range and operation approaching the supply voltage limit.
Using feedback control in EPS systems has several unique and demanding requirements, including satisfactory performance in a broad speed range and operation near or at the supply voltage limit. Unlike other motor control systems, such as those in high power applications, DC bus voltage in EPS systems is supplied by a vehicle battery and is usually relatively low (typically less than 20V). In addition, the motor in an EPS system is typically sized as efficiently as possible to deliver steady state power requirements. Therefore, EPS motors frequently operate near or at the voltage limit. Further, for closed-loop current feedback motor control architectures, the control system is designed to track current step commands with near-zero steady state error. Two integrators, one for each current loop, may be utilized for this purpose. The current controller applies a suitable transformation on the reference and measured currents, in order to obtain voltage commands, which are then applied to the motor via a voltage source inverter (VSI).
Some current control systems are designed as quasi-linear feedback architectures expected to satisfy a given set of linear system performance metrics. However, non-linear constraints associated with the actuator can cause the linear feedback control loop to destabilize when the actuator limits are reached, resulting in degraded overall system performance. This degraded performance can manifest itself both in terms of tracking behavior, torque ripple and audible noise. One such nonlinearity in the motor control system is that the total voltage applied to the motor is limited by the amount of voltage available at the input of the VSI. If the linear control loop computes voltage commands that result in an overall voltage command magnitude that is beyond the output of the battery, the voltage commands may need to be limited.
An issue that arises due to the presence of such a plant input saturation nonlinearity is that when the controller computes voltage commands (transient or steady-state) beyond the voltage limit of the system, the voltage commands are limited to the maximum available battery voltage and the states (or element of the controller that has memory), such as that of an integrator, become incorrect, because they do not conform to the non-limited or pre-limited voltage commands computed originally. If the saturated condition lasts for a long period of time, the states of the controller may become highly incorrect. When the system returns to the linear operating range, the states can return to correct values after a certain amount of time, which is dependent on how long the saturation condition lasted as well as the post saturation conditions. This situation, referred to as controller windup, can produce poor overall control system performance and instability.
Accordingly, it is desirable to include an anti-windup (AW) to improve performance of the motor control system, and in turn the EPS system using the motor control system.